The automotive industry is attempting to replace metal and other materials with plastics, such as rigid polypropylene, which are light and can be readily molded into complex shapes. However, the bonding of two parts or substrates of rigid polypropylene which is the preferred plastic does present problems.
It is generally believed that in order to develop an acceptable and effective bond between two substrates of rigid polypropylene a "pretreatment" of the substrates must first be effected. The pretreatment can consist of exposure to corona discharge/static discharge, flame pretreatments are not always safe, effective and efficient. In addition, they can also be time-consuming and decrease production rates as a consequence.
Certain automotive production lines have chosen to eliminate the pretreatment/adhesive process altogether and to bond the polypropylene substrates by ultrasonic welding. However, this method is expensive because of the relatively high reject rates of the sonically welded polypropylene parts. Furthermore, the purely mechanical bonds formed by sonic welding are not always effective.
It would be advantageous to have adhesive compositions for bonding polypropylene that eliminated the need for pretreatment of the substrate and the use of corona discharge equipment and chemical primers.